Sometimes I like mine best near the edges, but most times, I find the what seems the best vibration management, in my setting, something like this:

the back pair in the gap between the wood frame and the base inset and in from the corners... pretty close to under the treble knobs. This is sort of between the weight of the transformers and chokes.

On the front, right now I get the most from having them almost exactly under the front power tubes...just a little toward the input tubes.

This setting seems to give me what i think of as good tube sound, unimpaired by vibration...deep/tight real sounding bass, rich textural mids, and smooth highs.

I guess it depends on tastes, but really it may have more to do with how our individual amps vibrate in our environments. Getting the vibration out of the equation seems to me to equal the beauty sound with no holes in the presentation spectrum.

Interesting. I have tried those locations and I get the widest, cleanest sound at the corners (but inside a bit as I can't quite get them right on the corners). I think it's quite true that the environment of the amp is an influence. I have mine on top a 4" maple platform! On the top shelf of a Mapleshade Samson rack.

I don't know whether to be happy I cannot tell a difference in these locations, or to believe that in my setup there is no difference, or whether I was just too happy with morning latté and music. In any case, I use 4 just inside at a 45-degree angle from each of the feet on the amps and Ultra. I can easily discern not having the IsoCups, but am unable to discern any difference on the positional changes.

Lon, LOL, I love that explanation! So, my IsoCups are are just cognitive decor. I am good with that.

Speaking of the racks, I just ordered a short ZRACK at 32 inches to complement my two taller ones. I plan to put it in the middle, and have it hold some of the smaller components that are pushing the capacity limits of the taller racks. [orangecrush, I have no doubt your stands are great, but my post and, I suspect, Lon's reply, were written with some humor.]